High-strength galvanized steel sheet, high strength member, and method for manufacturing the same

ABSTRACT

A high-strength galvanized steel sheet includes a steel sheet containing a predetermined component element, a mass ratio of a content amount of Si to a content amount of Mn in the steel (Si/Mn) being 0.2 or more, and a steel structure in which an average grain size of inclusions existing in an area extending from a surface to a position of ⅓ of a sheet thickness is 50 μm or less, and an average nearest distance between ones of the inclusions is 20 μm or more; and a galvanized layer provided on a surface of the steel sheet, in which an amount of diffusible hydrogen contained in the steel is less than 0.25 mass ppm, oxides containing predetermined elements in an outer layer portion of the steel sheet account for 0.010 g/m2 or more per one surface, and a tensile strength is 1100 MPa or more.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2019/014235, filedMar. 29, 2019, which claims priority to Japanese Patent Application No.2018-068995, filed Mar. 30, 2018 and Japanese Patent Application No.2019-037383, filed Mar. 1, 2019, the disclosures of these applicationsbeing incorporated herein by reference in their entireties for allpurposes.

FIELD OF THE INVENTION

The present invention relates to a high-strength galvanized steel sheet,a high strength member, and a method for manufacturing the same that areexcellent in plating ability and bendability, and that is suitable forbuilding materials and automotive collision-resistant parts, and amethod for manufacturing the same.

BACKGROUND OF THE INVENTION

In these days when crash safety and fuel efficiency improvement ofautomobiles are strongly required, the strength increase of steel sheetsthat are materials of parts is being advanced. Further, in view of thefact that automobiles are being widely spread on a global scale andautomobiles are used for various uses in diverse areas and climates,steel sheets that are materials of parts are required to have highantirust properties.

In general, when the strength of a steel sheet is enhanced, theformability of the steel sheet is reduced. In particular, a steel sheetprovided with plating tends to have poorer formability than a steelsheet not provided with plating.

If a large amount of an alloying element is incorporated in order toincrease strength, it is difficult for a good quality plating film to beformed on the steel sheet. Further, it is known that, if plating of Zn,Ni, or the like is provided, hydrogen that enters during themanufacturing process is less likely to be released from the interior ofthe steel.

Steel sheets having excellent bendability have thus far been developed.Based on the features of the method for forming such a steel sheet, howto design a location that is exposed to most severe forming conditionsat the time of bending, that is, a location where stress is concentratedis presented as a solution to an issue. In particular, in the case of asteel sheet containing two or more kinds of steel structures withdifferent hardnesses, it is likely that deformation will concentrate anddefects of microvoids will occur at the interface between steelstructures, and consequently bendability is degraded.

Also the control of an atmosphere in the furnace of an annealing-platingstep is attempted in order to deposit good quality plating.

In Non-Patent Literatures 1 and 2, while the steel structure of a steelsheet contains ferrite and martensite, a steel structure of ferrite andmartensite is temporarily produced and then tempering is performed tosoften the martensite, and bendability is improved.

Patent Literature 1 discloses a high-strength steel sheet in which astructure homogeneity index given by a standard deviation of Rockwellhardness of a surface of a steel sheet and serving as an indexindicating the homogeneity of the steel sheet is 0.4 or less and that isgood in ductility and bendability and has a maximum tensile stress of900 MPa or more, and a method for manufacturing the same. Thisliterature provides a technique obtained as a result of improving, as afactor influencing bendability, the heterogeneity of a solidifiedstructure at the time of casting, and presents, by this method, a steelsheet that has a maximum tensile stress of 900 MPa or more and isexcellent in bendability.

In Patent Literature 1, at this time, the interior of an annealingfurnace of a continuous galvanizing line is set to an atmosphere havinga hydrogen concentration of 1 to 60 vol % and containing N₂, H₂O, O₂,and incidental impurities as the balance, and the logarithm of thepartial pressure of water and the partial pressure of hydrogen in theatmosphere, log (P_(H2O)/P_(H2))₁ is prescribed to−3≤log(P_(H2O)/P_(H2))≤−0.5 in order to ensure good quality platingability.

Patent Literature 2 provides a dual-phase steel sheet that contains 50%or more of bainite and 3 to 30% of retained austenite and in which theratio between the hardness Hvs of an outer layer of the steel sheet andthe hardness Hvb of a portion of ¼ of the thickness of the steel sheetis prescribed to 0.35 to 0.90. Further, annealing is performed in anatmosphere in which log(partial pressure of water/partial pressure ofhydrogen) is −3.0 to 0.0, and thereby plating ability is ensured in ahigh alloy system.

Patent Literature 3 ensures bendability by prescribing a decarburizedferrite layer, and discloses, as a technique for manufacturing a platedsteel sheet, a method of adjustment to an atmosphere containing 2 to 20vol % of hydrogen and the balance containing nitrogen and impurities andhaving a dew-point temperature of more than −30° C. and 20° C. or less.

PATENT LITERATURE

-   Patent Literature 1: JP 2011-111670 A-   Patent Literature 2: JP 2013-163827 A-   Patent Literature 3: JP 2017-048412 A

Non-Patent Literature

-   Non-Patent Literature 1: Kohei Hasegawa, and five others, “980    MPa-kyu Cho-ko-kyodo Kohan no Mage-kako-sei ni Oyobosu    Kinzoku-soshiki no Eikyo” (Influence of Metal Structure on the    Bending Formability of an Ultrahigh-strength Steel Sheet of the    980-MPa Class), CAMP-ISIJ, vol. 20 (2007), p. 437, published by The    Iron and Steel Institute of Japan-   Non-Patent Literature 2: Nobuyuki Nakamura, and three others,    “Cho-ko-kyodo Reien Kohan no Nobi-furanji-seikei-sei ni Oyobosu    Soshiki no Eikyo” (Influence of Structure on the Stretch Flange    Moldability of an Ultrahigh-strength Cold Rolled Steel Sheet),    CAMP-ISIJ, vol. 13 (2000), p. 391, published by The Iron and Steel    Institute of Japan

SUMMARY OF THE INVENTION

Thus far, to improve the bendability of a steel sheet, mainly theoptimization of steel structure has been made; however, this providesonly a certain level of improvement, and further improvement isrequired. Further, it is presumed that, in the case where a highalloy-based steel sheet is subjected to plating, hydrogen in theatmosphere in the plating step becomes hydrogen in steel remaining inthe steel sheet product. It is presumed that improvement in bendabilityis hindered by the hydrogen in steel. It is also necessary to achieveboth improvement in bendability and plating ability.

Aspects of the present invention improve the bendability of a platedsteel sheet from a new point of view, and an object according to aspectsof the present invention is to provide a high-strength galvanized steelsheet and a high strength member excellent in plating ability andbendability, and a method for manufacturing them.

The high strength referred to in the present specification means thattensile strength (TS) is 1100 MP or more.

The present inventors conducted extensive studies in order to solve theissue mentioned above. As a result, it has been found out that, toimprove the bendability of a plated steel sheet, it is necessary toappropriately adjust the amount of hydrogen remaining in the steel inaddition to the existence state of inclusions existing from the vicinityof the outer layer in the sheet thickness to near the center of thesheet thickness. Further, it has been found out that a high-strengthgalvanized steel sheet having good bendability and plating ability isobtained by, in addition to controlling inclusions and adjusting theamount of hydrogen in the steel, setting the steel sheet to a specificchemical composition, adjusting particularly the mass ratio of thecontent of Si to the content of Mn in the steel (Si/Mn) to apredetermined range, and making adjustment so that the amount of oxidescontaining predetermined elements and existing in an outer layer portionof the steel sheet is within a predetermined range.

Further, it has been found out that a high-strength galvanized steelsheet according to aspects of the present invention can be manufacturedby appropriately adjusting conditions of manufacturing steps, such asconditions of an atmosphere in the furnace during recrystallizationannealing. In particular, in the course of studies on manufacturingconditions of a galvanized steel sheet according to aspects of thepresent invention, the present inventors have found for the first timethat the plating ability of the galvanized steel sheet can bedramatically improved by controlling the dew-point temperature of theatmosphere in the furnace in an annealing step to a specific range. Thisis presumed to be because, by controlling the dew-point temperature, theamount of oxides in an outer layer portion of the steel sheet has beensuccessfully adjusted to within a predetermined range and thereby theexternal oxidation of Si and Mn has been successfully suppressed.Specifically, aspects of the present invention provide the following.

[1] A high-strength galvanized steel sheet including:

a steel sheet having a chemical composition containing a steelcomposition containing, in mass %,

C: 0.08% or more and 0.20% or less,

Si: less than 2.0%,

Mn: 1.5% or more and 3.5% or less,

P: 0.02% or less,

S: 0.002% or less,

Al: 0.10% or less, and

N: 0.006% or less,

a mass ratio of a content of Si to a content of Mn in the steel (Si/Mn)being 0.2 or more, and the balance: Fe and incidental impurities, and

a steel structure in which an average grain size of inclusionscontaining at least one of Al, Si, Mg, and Ca and existing in an areaextending from a surface to a position of ⅓ of a sheet thickness is 50μm or less, and an average nearest distance between the inclusions is 20μm or more; and

a galvanized layer provided on a surface of the steel sheet and having acoating weight per one surface of 20 g/m² or more and 120 g/m² or less,

in which an amount of diffusible hydrogen contained in the steel is lessthan 0.25 mass ppm,

oxides containing at least one or more elements selected from Fe, Si,Mn, Al, P, B, Nb, Ti, Cr, Mo, V, Cu, and Ni and existing in an outerlayer portion of the steel sheet within 100 μm from a surface of anunderlying steel sheet immediately below the galvanized layer accountfor 0.010 g/m² or more per one surface, and

a tensile strength is 1100 MPa or more.

[2] The high-strength galvanized steel sheet according to [1], in whichthe chemical composition further contains, in mass %, at least one of(1) to (3) below,

(1) one or more of Ti, Nb, V, and Zr: 0.005% or more and 0.1% or less intotal,

(2) one or more of Mo, Cr, Cu, and Ni: 0.01% or more and 0.5% or less intotal, and

(3) B: 0.0003% or more and 0.005% or less.

[3] The high-strength galvanized steel sheet according to [1] or [2], inwhich the chemical composition further contains, in mass %, at least oneof Sb: 0.001% or more and 0.1% or less and Sn: 0.001% or more and 0.1%or less.

[4] The high-strength galvanized steel sheet according to any one of [1]to [3], in which the chemical composition further contains, in mass %,Ca: 0.0005% or less.

[5] The high-strength galvanized steel sheet according to any one of [1]to [4], in which the steel structure contains 30% or more and 85% orless of martensite, 60% or less (including 0%) of ferrite, 15% or less(including 0%) of bainite, and less than 5% (including 0%) of retainedaustenite in terms of area ratio, and an average grain size of ferriteis 15 μm or less.

[6] A method for manufacturing a high-strength galvanized steel sheet,including:

a casting step of casting steel having the chemical compositionaccording to any one of [1] to [4] under a condition where a flowvelocity of molten steel at a solidification interface in vicinity of ameniscus of a casting mold is 16 cm/second or more, and producing asteel raw material;

a hot rolling step of hot rolling the steel raw material after thecasting step;

a pickling step of pickling a steel sheet after the hot rolling step;

a cold rolling step of cold rolling the steel sheet after the picklingstep at a rolling reduction ratio of 20% or more and 80% or less;

an annealing step of heating the steel sheet after the cold rolling stepin a continuous annealing line at an annealing temperature of 740° C. ormore and (Ac3+20°) C. or less in an atmosphere in the furnace in which ahydrogen concentration in a temperature region of 500° C. or more ismore than 0 vol % and 12 vol % or less and a dew-point temperature in atemperature region of 740° C. or more is −25° C. or more, and thenperforming cooling at an average cooling rate of 3° C./s or more fromthe annealing temperature to at least 600° C.; and

a plating step of subjecting the steel sheet after the annealing step toplating treatment, and after the plating treatment, performing coolingat an average cooling rate of 3° C./s or more through a temperatureregion of 450° C. to 250° C.

[7] The method for manufacturing a high-strength galvanized steel sheetaccording to [6], further including, after the plating step, a widthtrimming step of performing width trimming.

[8] The method for manufacturing a high-strength galvanized steel sheetaccording to [6] or [7], further including, after the annealing step orafter the plating step, a post-treatment step of performing heating in atemperature region of 50 to 400° C. for 30 seconds or more in anatmosphere with a hydrogen concentration of 5 vol % or less and adew-point temperature of 50° C. or less.

[9] The method for manufacturing a high-strength galvanized steel sheetaccording to any one of [6] to [8], in which alloying treatment isperformed immediately after the plating treatment in the plating step.

[10] A high strength member, obtained by subjecting the high-strengthgalvanized steel sheet according to any one of [1] to [5] to at leasteither one of forming and welding.

[11] A method for manufacturing a high strength member, including a stepof performing at least either one of forming and welding on ahigh-strength galvanized steel sheet manufactured by the method formanufacturing a high-strength galvanized steel sheet according to anyone of [6] to [9].

According to aspects of the present invention, a high-strengthgalvanized steel sheet and a high strength member excellent in platingability and bendability and a method for manufacturing them can beprovided. In the case where a high-strength galvanized steel sheetaccording to aspects of the present invention is used for a frameworkmember of an automobile body, the high-strength galvanized steel sheetcan make a great contribution to improvement in collision safety andweight reduction.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a diagram showing an example of relationship between theamount of diffusible hydrogen in steel and R/t.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereafter, the embodiments of the present invention will be described.Here, the present invention is not limited to the embodiments describedbelow.

A high-strength galvanized steel sheet according to aspects of thepresent invention includes a steel sheet and a galvanized layer formedon a surface of the steel sheet. First, the chemical composition of thesteel sheet (a steel composition) will be described. In the descriptionof the chemical composition of the steel sheet, “%” that is the unit ofthe content of a component means “mass %”.

C: 0.08% or more and 0.20% or less

C is an effective element to increase the strength of the steel sheet,and contributes to strength increase by forming martensite, which is ahard phase of steel structure. Further, depending on the manufacturingmethod, C contributes to strength increase also by forming a fine alloycompound or an alloy carbonitride together with a carbide-formingelement such as Nb, Ti, V, or Zr. To obtain these effects, the contentamount of C needs to be set to 0.08% or more. On the other hand, if thecontent of C is more than 0.20%, martensite is hardened excessively, andbending formability tends not to be improved even if inclusions or theamount of hydrogen in steel is controlled. Thus, the content of C is setto 0.08% or more and 0.20% or less. From the viewpoint of stablyachieving a TS of 1100 MPa or more, the content of C is preferably 0.09%or more.

Si: less than 2.0%

Si is an element contributing mainly to strength increase by solidsolution strengthening; and experiences relatively small reduction inductility with respect to strength increase, and contributes to not onlystrength but also improvement in balance between strength and ductility.Improvement in ductility leads to improvement in bendability. On theother hand, Si is likely to form Si-based oxides on the surface of thesteel sheet, and may be a cause of bare spot; furthermore, if Si iscontained excessively, significant scales are formed during hot rollingand scale flaws are marked on the surface of the steel sheet;consequently, surface quality may be deteriorated, and pickling abilitymay be reduced. Thus, it is sufficient to add Si only an amountnecessary to ensure strength; from the viewpoint of plating ability, thecontent of Si is set to less than 2.0%. The content amount of Si ispreferably 1.8% or less. The lower limit of the content of Si is notparticularly prescribed; however, if the content of Si is less than0.001%, control in manufacturing tends to be difficult; thus, thecontent of Si is preferably set to 0.001% or more. It is sufficient toadd Si only an amount necessary to ensure, and from the viewpoint ofobtaining effects of ensuring strength effectively, the content of Si ismore preferably 0.3% or more, still preferably 0.4% or more, andparticularly preferably 0.5% or more.

Mn: 1.5% or more and 3.5% or less

Mn is effective as an element contributing to strength increase by solidsolution strengthening and martensite formation, and to obtain thiseffect, the content of Mn needs to be set to 1.5% or more. The contentof Mn is preferably 1.9% or more. On the other hand, if the content ofMn is more than 3.5%, unevenness is likely to occur in the steelstructure due to segregation or the like of Mn and formabilitydecreases, and Mn is likely to be externally oxidized as oxides orcomposite oxides on the surface of the steel sheet, and may be a causeof bare spot. Thus, the content of Mn is set to 3.5% or less.

P: 0.02% or less

P is an effective element contributing to the strength increase of thesteel sheet by solid solution strengthening, but on the other handinfluences plating ability. In particular, P causes degradation inwettability with the steel sheet and reduction in the alloying rate of acoating layer, and there is great influence particularly in a high alloysystem whereby a high-strength steel sheet is obtained. Thus, thecontent amount of P is set to 0.02% or less. The content of P ispreferably 0.01% or less. The lower limit of the content of P is notparticularly prescribed; however, if the lower limit is less than0.0001%, a reduction in production efficiency and dephosphorization costincrease are brought about in the manufacturing process; thus, thecontent of P is preferably set to 0.0001% or more.

S: 0.002% or less

S is likely to form sulfide-based inclusions in the steel. Inparticular, in the case where a large amount of Mn is added for strengthincrease, MnS-based inclusions are likely to be formed. This is a causeof impairing bendability; in addition, S causes hot brittleness, andgives adverse effect on the manufacturing process; thus, the amount of Sis preferably reduced as much as possible. In accordance with aspects ofthe present invention, up to 0.002% is acceptable. The lower limit ofthe content of S is not particularly prescribed; however, if the lowerlimit is less than 0.0001%, a reduction in production efficiency andcost increase are brought about in the manufacturing process; thus, thecontent of S is preferably set to 0.0001% or more.

Al: 0.10% or less

Al is added as a deoxidizer. In the case where Al is added as adeoxidizer, it is preferable that 0.001% or more of Al be contained inorder to obtain this effect. On the other hand, if the content of Al ismore than 0.10%, inclusions are likely to be formed during themanufacturing process, and bendability is degraded. Thus, the content ofAl is set to 0.10% or less, and is preferably 0.08% or less as sol. Alin the steel.

N: 0.006% or less

If the content of N is more than 0.006%, excessive nitrides are producedin the steel and formability is reduced, and the deterioration of thesurface quality of the steel sheet may be caused. Hence, the content ofN is set to 0.006% or less, and preferably 0.005% or less. If there isferrite, although the content amount is preferably as small as possiblefrom the viewpoint of improving ductility by refining ferrite, suchamounts reduce production efficiency and increase cost in themanufacturing process; thus, the content of N is preferably set to0.0001% or more.

The mass ratio of the content of Si to the content of Mn in the steel(Si/Mn) is 0.2 or more.

To obtain excellent plating ability, the control of elements that arelikely to be oxidized in the steel is important; thus, from theviewpoint of suppressing the external oxidation of Mn, an aim is to formSi—Mn composite oxides in the interior of the steel sheet. If the massratio of the content of Si to the content of Mn in the steel (Si/Mn) isless than 0.2, a sufficient amount of oxides are not obtained in anouter layer portion of the steel sheet within 100 μm from the surface ofthe underlying steel sheet immediately below the galvanized layer(hereinafter, also referred to as simply “the surface of the underlyingsteel sheet”). Thus, the mass ratio of the content of Si to the contentof Mn in the steel (Si/Mn) is set to 0.2 or more. From the viewpoint ofobtaining a sufficient amount of oxides in the outer layer portion ofthe steel sheet, the mass ratio of the content of Si to the content ofMn in the steel (Si/Mn) is preferably 0.20 or more, and more preferably0.25 or more. The upper limit of the mass ratio of the content of Si tothe content of Mn in the steel (Si/Mn) is not particularly prescribed,but is often 1.3 or less in the chemical composition according toaspects of the present invention.

The steel according to aspects of the present invention basicallycontains the chemical composition mentioned above, and the balance isiron and incidental impurities. In the chemical composition mentionedabove, the components mentioned below may be further contained asarbitrary components to the extent that the action according to aspectsof the present invention is not impaired. In the case where any of thearbitrary elements mentioned below is contained at less than the lowerlimit value mentioned below, it is assumed that the arbitrary componentis contained as an incidental impurity. Further, in the chemicalcomposition, Mg, La, Ce, Bi, W, and Pb may be contained as incidentalimpurities up to 0.002% in total.

The chemical composition mentioned above may further contain, asarbitrary components, at least one of (1) to (3) below in mass %.

(1) one or more of Ti, Nb, V, and Zr: 0.005% or more and 0.1% or less intotal,

(2) one or more of Mo, Cr, Cu, and Ni: 0.01% or more and 0.5% or less intotal, and

(3) B: 0.0003% or more and 0.005% or less.

Ti, Nb, V, and Zr form, together with C or N, carbides or nitrides (alsopossibly carbonitrides). These elements contribute to the strengthincrease of the steel sheet by being formed in fine precipitates. Inparticular, by precipitating these elements in soft ferrite, thestrength of the soft ferrite is enhanced, and the strength differencewith martensite is reduced; this effect contributes to improvement innot only bendability but also stretch flangeability. Further, theseelements have the action of refining the structure of a hot rolled coil;thus, contribute to strength increase and improvement in formabilitysuch as bendability also by refining the steel structure after coldrolling and annealing subsequent to the hot rolling. From the viewpointof obtaining this effect, it is preferable that one or more of Ti, Nb,V, and Zr be contained at 0.005% or more in total. However, excessiveaddition increases deformation resistance during cold rolling andinhibits productivity, and the presence of surplus or coarseprecipitates tends to reduce the ductility of ferrite and reduce theductility or bendability of the steel sheet. Hence, it is preferablethat one or more of Ti, Nb, V, and Zr be contained 0.1% or less intotal.

The elements of Mo, Cr, Cu, and Ni enhance hardenability and facilitategeneration of martensite, and are therefore elements contributing tostrength increase. To obtain these effects, the lower limit mentionedabove of 0.01% is prescribed as a preferred lower limit. Excessiveaddition of Mo, Cr, Cu, and Ni leads to the saturation of the effect andcost increase; further, Cu induces cracking during hot rolling, and is acause of the occurrence of surface flaws. Thus, it is preferable thatone or more of Mo, Cr, Cu, and Ni be contained 0.5% or less in total. Nihas the effect of impeding the occurrence of surface flaws resultingfrom Cu addition, and is therefore preferably added in a simultaneousmanner when Cu is added. In particular, the content of Ni is preferably½ or more of the amount of Cu.

Also for B, in addition to the lower limit mentioned above for obtainingthe effect of suppressing ferrite formation occurring during anannealing cooling process, an upper limit is provided for the excessiveaddition due to the saturation of the effect. Excessive hardenabilityhas also a disadvantage such as weld cracking during welding. Thus,content of B is preferably set to 0.0003% or more and 0.005% or less.

The chemical composition mentioned above may further contain, as anoptional component, the following component.

At least one of Sb: 0.001% or more and 0.1% or less and Sn: 0.001% ormore and 0.1% or less

Sb and Sn are effective elements to suppress decarburization,denitrification, deboronization, etc., and suppress the strengthreduction of the steel sheet; thus, the content of each element ispreferably 0.001% or more. However, excessive addition reduces surfacequality; thus, the upper limit of the content of each element ispreferably set to 0.1%.

Ca: 0.0005% or less

When a small amount of Ca is added, the effect of spheroidizing theshapes of sulfides and improving the bendability of the steel sheet isobtained. On the other hand, if Ca is added excessively, Ca formssulfides or oxides in the steel excessively, and reduces theformability, particularly bendability, of the steel sheet; thus, thecontent of Ca is preferably set to 0.0005% or less. The lower limit ofthe content of Ca is not particularly prescribed; however, in the casewhere Ca is contained, the content of Ca is often 0.0001% or more.

Next, the steel structure of the steel sheet is described.

In the steel structure, the average grain size of inclusions containingat least one of Al, Si, Mg, and Ca and existing in an area extendingfrom a surface to a position of ⅓ of the sheet thickness is 50 μm orless, and the average nearest distance between inclusions is 20 μm ormore. Bendability can be improved when the average grain size ofinclusions and the average nearest distance between inclusions areadjusted to the ranges mentioned above and the amount of diffusiblehydrogen in the steel is set in a specific range. In the measurement ofthe nearest distance between inclusions, inclusions other thaninclusions containing at least one of Al, Si, Mg, and Ca are notincluded.

The average grain size of inclusions is 50 μm or less, preferably 30 μmor less, and more preferably 20 μm or less. The average grain size ofinclusions is preferably as small as possible; thus, the lower limit isnot particularly prescribed, but is often 1 μm or more.

The average nearest distance of inclusions is 20 μm or more, preferably30 μm or more, and more preferably 50 μm or more. As for the averagenearest distance of inclusions, the upper limit is not particularlyprescribed, but is often 500 μm or less.

The average grain size of inclusions and the average nearest distancebetween inclusions are measured by methods described in Examples.

Further, in accordance with aspects of the present invention, the steelstructure of a steel sheet preferably contains 30% or more and 85% orless of martensite, 60% or less (including 0%) of ferrite, 15% or less(including 0%) of bainite, and less than 5% (including 0%) of retainedaustenite in terms of area ratio, and an average grain size of ferriteis 15 μm or less.

Martensite: 30% or more and 85% or less

Martensite is a hard structure, and is effective and essential toenhance the strength of the steel sheet. In order to ensure a tensilestrength (TS) of 1100 MPa or more, the amount of martensite ispreferably set to 30% or more in terms of area ratio. From the viewpointof ensuring TS stably, the amount of martensite is preferably set to 45%or more. The martensite herein includes autotempered martensite that isself-tempered during manufacturing and, depending on the circumstances,tempered martensite that is tempered by a subsequent heat treatment.From the viewpoint of the balance between bendability and strength, theamount of martensite is preferably set to 85% or less.

Ferrite: 60% or less (including 0%)

In the case where heat treatment and a step of providing plating areperformed in an atmosphere where hydrogen exists, hydrogen enters theinterior of the steel and remains in the steel. As a technique forreducing the amount of hydrogen in the steel of the end product as muchas possible, ferrite and bainite having BCC structures are formed in thesteel structure before providing plating. This utilizes the fact thatthe solid solubility of hydrogen is smaller in ferrite and bainitehaving BCC structures than in austenite having an FCC structure.Further, soft ferrite improves the ductility of the steel sheet, andimproves bendability. However, if ferrite exceeds 60%, strength cannotbe ensured; thus, a preferred upper limit is set to 60%. Ferrite oftenaccounts for 2% or more.

The average grain size of ferrite is preferably 15 μm or less. Thesmaller the ferrite grain size is, the more the generation and linkageof voids on the bending surface can be suppressed, and the more thebendability can be enhanced. The average grain size of ferrite is morepreferably 10 μm or less, and still more preferably 4 μm or less.

Bainite: 15% or less (including 0%)

Bainite contributes to improvement in bendability, and may therefore becontained; however, if bainite is contained excessively, desiredstrength is not obtained; thus, the amount of bainite is preferably setto 15% or less. Bainite often accounts for 2% or more.

Retained austenite accounting for less than 5% (including 0%)

Austenite is an fcc phase; as compared to ferrite (a bcc phase),austenite has high ability of occluding hydrogen, and is diffused slowlyin the steel and is therefore likely to remain in the steel. Further, inthe case where the retained austenite experiences strain-inducedtransformation to martensite, there is a concern that the amount ofdiffusible hydrogen in the steel will be increased. Thus, in accordancewith aspects of the present invention, retained austenite preferablyaccounts for less than 5%.

The steel structure occasionally contains precipitates of pearlite,carbides, etc. in the balance, as structures other than the structures(phases) mentioned above; these can be permitted as long as they accountfor 10% or less as the total area ratio in a position of ¼ of the sheetthickness from the surface of the steel sheet. The amount of these otherstructures is preferably set to 5% or less (including 0%).

The inclusions and the area ratios of the steel structure mentionedabove are found by methods described in Examples.

Next, the galvanized layer is described. For the galvanized layer, thecoating weight per one surface is 20 to 120 g/m². If the coating weightis less than 20 g/m², it is difficult to ensure corrosion resistance.Thus, the coating weight is set to 20 g/m² or more, preferably 25 g/m²or more, and more preferably 30 g/m² or more. On the other hand, if thecoating weight is more than 120 g/m², plating peeling resistance isdegraded. Thus, the coating weight is 120 g/m² or less, preferably 100g/m² or less, and more preferably 80 g/m² or less.

The composition of the galvanized layer is not particularly limited, andmay be a common composition. For example, in the case of a hot-dipgalvanized layer or an alloyed hot-dip galvanized layer, the compositionis generally a composition containing Fe: 20 mass % or less and Al:0.001 mass % or more and 1.0 mass % or less, further containing one ortwo or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu,Li, Ti, Be, Bi, and REMs at 0 mass % or more and 3.5 mass % or less intotal, and containing the balance containing Zn and incidentalimpurities. In accordance with aspects of the present invention, it ispreferable to have a hot-dip galvanized layer in which the coatingweight per one surface is 20 to 120 g/m² or an alloyed hot-dipgalvanized layer in which the hot-dip galvanized layer is furtheralloyed. In the case where the coating layer is a hot-dip galvanizedlayer, the content of Fe in the coating layer is preferably less than 7mass %; in the case where the coating layer is an alloyed hot-dipgalvanized layer, the content of Fe in the coating layer is preferably 7to 20 mass %.

In the high-strength galvanized steel sheet according to aspects of thepresent invention, the amount of diffusible hydrogen in the steelobtained by measurement by a method described in Examples is less than0.25 mass ppm. Diffusible hydrogen in the steel degrades bendability. Ifthe amount of diffusible hydrogen in the steel is 0.25 mass ppm or more,bendability is deteriorated even if inclusions and steel structure areproduced properly.

In accordance with aspects of the present invention, it has beenrevealed that a stable improvement effect is provided by setting theamount of diffusible hydrogen in the steel to less than 0.25 mass ppm.The amount of diffusible hydrogen in the steel is preferably 0.20 massppm or less, and more preferably 0.15 mass ppm or less. The lower limitis not particularly limited, but is preferably as small as possible;thus, the lower limit is 0 mass ppm. In accordance with aspects of thepresent invention, it is necessary that, before subjecting the steelsheet to forming or welding, diffusible hydrogen in the steel accountfor less than 0.25 mass ppm. Note that, if the amount of diffusiblehydrogen in the steel measured by using a sample cut out from a product(a member) that is obtained after subjecting the steel sheet to formingor welding and that is placed in a common usage environment is less than0.25 mass ppm, the amount of diffusible hydrogen in the steel can beregarded as having been less than 0.25 mass ppm also before the formingor the welding.

In the high-strength galvanized steel sheet according to aspects of thepresent invention, oxides containing at least one or more elementsselected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, V, Cu, and Ni andexisting in an outer layer portion of the steel sheet within 100 μm fromthe surface of the underlying steel sheet immediately below thegalvanized layer account for 0.010 g/m² or more per one surface. Inorder that a hot-dip galvanized steel sheet in which Si and a largeamount of Mn are added in steel can be caused to satisfy good externalappearance and plating adhesiveness at the time of forming, it isnecessary to control the amount of oxides in an outer layer of the basesteel immediately below the coating layer, which oxides can be startingpoints of cracking during high degree of forming. Thus, in accordancewith aspects of the present invention, the dew-point temperature iscontrolled in an annealing step described later in order to control anoxygen potential. Thereby, the oxygen potential is enhanced;accordingly, Si, Mn, etc., which are easily oxidizable elements,experience internal oxidation in advance immediately before plating, andthe activity of Si and Mn in an outer layer portion of the base steel isreduced; thus, external oxidation is suppressed, and consequentlyimprovement in plating external appearance and adhesiveness is achieved.This effect of improvement is made significant by causing oxidescontaining at least one or more elements selected from Fe, Si, Mn, Al,P, B, Nb, Ti, Cr, Mo, V, Cu, and Ni and existing in an outer layerportion of the steel sheet within 100 μm from the surface of theunderlying steel sheet to exist at 0.010 g/m² or more per one surface.Such oxides preferably account for 0.060 g/m² or more. On the otherhand, even if such oxides are caused to exist at more than 0.30 g/m²,the effect is saturated; thus, the amount of such oxides is preferablyset to 0.30 g/m² or less. The oxides containing at least one or moreelements selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, V, Cu, andNi and existing in an outer layer portion of the steel sheet within 100μm from the surface of the underlying steel sheet are measured by amethod described in Examples.

The high-strength galvanized steel sheet according to aspects of thepresent invention has high tensile strength (TS). Specifically, thetensile strength (TS) measured by a method described in Examples is 1100MPa or more. The sheet thickness of the high-strength galvanized steelsheet according to aspects of the present invention is not particularlylimited, but is preferably set to 0.5 mm or more and 3 mm or less.

Next, a method for manufacturing a high-strength galvanized steel sheetaccording to aspects of the present invention is described. Themanufacturing method according to aspects of the present inventionincludes a casting step, a hot rolling step, a pickling step, a coldrolling step, an annealing step, and a plating step. Each step will nowbe described. The temperatures at the time of heating or cooling slabs(steel raw materials), steel sheets, etc. shown below mean, unlessotherwise stated, the surface temperatures of the slabs (the steel rawmaterials), the steel sheets, etc.

The casting step is a step of casting steel having the chemicalcomposition mentioned above under a condition where the flow velocity ofmolten steel at the solidification interface in the vicinity of themeniscus of the casting mold is 16 cm/s or more, and producing a steelraw material.

Manufacturing of steel raw material (slab (cast piece))

As the steel used in the manufacturing method according to aspects ofthe present invention, steel manufactured by a continuous castingmethod, generally called a slab, is used; this is for the purpose ofpreventing macrosegregation of alloy components; the manufacturing maybe performed also by an ingot casting, a thin slab casting method, orthe like.

In the case where continuous casting is performed, casting is performedunder a condition where the flow velocity of molten steel at thesolidification interface in the vicinity of the meniscus of the castingmold (hereinafter, also referred to simply as the flow velocity ofmolten steel) is 16 cm/s or more, from the viewpoint of controllinginclusions. The flow velocity of molten steel is preferably 17 cm/s ormore. By increasing the flow velocity of molten steel, it becomes easyto obtain a steel sheet according to aspects of the present invention;thus, the upper limit is not particularly prescribed; however, from theviewpoint of operating stability, the upper limit is preferably set to50 cm/s or less. “The vicinity of the meniscus of the casting mold”means the interface between powder used during continuous casting andmolten steel in the casting mold. In the case of ingot making, it ispreferable that inclusions be caused to sufficiently float up duringsolidification, the place where the inclusions float up and gather becut off, and the resulting piece be used for the next step.

A hot rolling step is a step of hot rolling the steel raw material afterthe casting step

After a steel slab has been manufactured, hot rolling may be performedby using any one of a conventional method in which the slab is reheatedafter having been cooled to room temperature, a method in which hotrolling is performed after the slab has been charged into a reheatingfurnace in the warm state without having been cooled to near-roomtemperature, a method in which hot rolling is performed immediatelyafter the slab has been subjected to heat retention for a short time,and a method in which hot rolling is performed directly on a cast piecein the hot state without problem.

The method of hot rolling is not particularly prescribed, but ispreferably performed under the following conditions.

It is preferable that the steel slab reheating temperature be 1100° C.or more and 1350° C. or less. The grain diameter of precipitates in thesteel slab tends to increase, and there is a disadvantage in that it isdifficult, for example, to achieve satisfactory strength throughprecipitation strengthening. Because there may be a case whereprecipitates having a large grain diameter have negative effects on theformation of a microstructure in the subsequent annealing process.Further, achieving a smooth steel sheet surface by heating in order toremove, for example, blowholes and defects from the surface of the slabthrough scale off so that there is a decrease in the number of cracksand in the degree of asperity on the surface of a steel sheet isadvantageous as product quality. From this viewpoint, the slab reheatingtemperature is prescribed. It is preferable that the reheatingtemperature be 1100° C. or more in order to realize such an effect. Onthe other hand, in the case where the heating temperature is more than1350° C., since there is an increase in austenite grain diameter, thereis an increase in the grain diameter of the steel structure of a finalproduct, which may result in a deterioration in the strength andbendability of a steel sheet, therefore, the preferable upper limit isprescribed.

In the hot rolling step including rough rolling and finish rolling,generally, a steel slab is made into a sheet bar by performing roughrolling, and the sheet bar is made into a hot-rolled coil by performingfinish rolling, however, there is no problem in the case where rollingis performed regardless of such a classification depending on, forexample, rolling mill capacity as long as a predetermined size isobtained.

The following are recommended as hot rolling conditions.

The finishing delivery temperature is preferably set in the range of800° C. or more and 950° C. or less. This is aimed at, by the setting to800° C. or more, making uniform the structure obtained in the hot rolledcoil and allowing also the structure of the end product to be uniform.If the structure is non-uniform, bendability tends to be reduced. On theother hand, in the case where the finishing delivery temperature is morethan 950° C., since there is an increase in the amount of oxides (scale)formed, there is an increase in the degree of asperity of an interfacebetween the base steel and the oxides, which may tend to result in adeterioration in the surface quality after pickling or cold rolling.Further, the crystal grain size is increased, and this tends to be acause of reducing the strength and the bendability of the steel sheet,like in a steel slab.

The hot rolled coil (hot rolled sheet) after completion of the hotrolling as described above is, for the purpose of the refinement andhomogenization of a microstructure, preferably started to be cooledwithin 3 seconds after finish rolling has been performed at an averagecooling rate of 10 to 250° C./s in a temperature region from [finishingdelivery temperature] to [finishing delivery temperature-100]° C., andcoiled in a temperature region from 450 to 700° C.

The pickling step is a step of pickling the steel sheet after the hotrolling step. Scales are dropped by pickling. Pickling conditions may beset as appropriate.

The cold rolling step is a step of cold rolling the steel sheet afterthe pickling step at a rolling reduction ratio of 20% or more and 80% orless.

The reason why the rolling reduction ratio is set to 20% or more is thatit is attempted to obtain uniform fine steel structure in the annealingstep subsequently performed. If the rolling reduction ratio is less than20%, it is likely that coarse grains will be produced and non-uniformstructure will be produced during annealing, and it is feared thatstrength and formability in the end product sheet will be reduced asdescribed above. For the upper limit, a high rolling reduction ratio maycause not only reduction in productivity due to the rolling load butalso shape failure; thus, the upper limit is set to 80%. It is alsopossible to perform pickling after cold rolling.

The annealing step is a step of heating the steel sheet after the coldrolling step in a continuous annealing line at an annealing temperatureof 740° C. or more and (Ac3+20°) C. or less, in an atmosphere in thefurnace in which a hydrogen concentration in a temperature region of500° C. or more is more than 0 vol % and 12 vol % or less and adew-point temperature in a temperature region of 740° C. or more is −25°C. or more, and then performing cooling at an average cooling rate of 3°C./s or more from the annealing temperature to at least 600° C. Thecooling stop temperature of cooling is not particularly limited. The Ac3transformation point (in the present specification, also written assimply Ac3) is calculated in the following way.

Ac3(° C.)=910−203(C)_(1/2)+44.7Si−30Mn−11P+700S+400Al+400Ti.

The atomic symbols in the equations above respectively denote thecontents (mass %) of the corresponding chemical elements, and where thesymbol of a chemical element which is not contained is assigned a valueof 0.

If the hydrogen concentration of the in-furnace atmosphere in thetemperature region of 500° C. or more is too high, there is a problemthat the amount of diffusible hydrogen in the steel prescribed inaccordance with aspects of the present invention becomes more than theupper limit; if the annealing temperature is too low, there is a problemof poor plating ability; thus, the hydrogen concentration of thein-furnace atmosphere in the temperature region of 500° C. or more isset to more than 0 vol % and 12 vol % or less. The hydrogenconcentration is preferably 10 vol % or less. From the viewpoint ofimproving plating ability, the hydrogen concentration is preferably 1vol % or more, and more preferably 3 vol % or more.

In an in-furnace atmosphere in which the dew-point temperature in thetemperature region of 740° C. or more is less than −25° C., in thepresent component system, composite oxides containing Si and Mn are lesslikely to be formed, and a sufficient amount of oxides are not obtainedin an outer layer portion of the steel sheet within 100 μm from thesurface of the underlying steel sheet; thus, the effect of suppressingthe external oxidation of Si and Mn is insufficient, and bare spotoccurs. Hence, the dew-point temperature in the annealing region of 740°C. or more is set to −25° C. or more. The upper limit of the dew-pointtemperature in the annealing region of 740° C. or more is notparticularly prescribed, but is preferably 10° C. or less in view of theconcern of the degradation of the roll due to pickup, etc.

The dew-point temperature for 740° C. or less, at which the influence onthe oxidation of Si and Mn is small, is not particularly prescribed, butis preferably −55° C. or more and 10° C. or less in view of the factthat it is very difficult to maintain a dew-point temperature of −55° C.or less from the viewpoint of ensuring the airtightness of the furnacebody and that dew-point temperatures of 10° C. or more have the concernof the degradation of the roll due to pickup, etc.

If the annealing temperature is too high, there is a problem that theamount of diffusible hydrogen in the steel prescribed in according toaspects the present invention exceeds the upper limit; if the annealingtemperature is too low, there is a problem that the tensile strengthprescribed in according to aspects the present invention is notobtained; thus, the annealing temperature is set to 740° C. or more and(Ac3+20°) C. or less.

If the average cooling rate from the annealing temperature to at least600° C. is too slow, there is a problem that an amount of martensite forobtaining desired characteristics cannot be ensured; thus, the averagecooling rate is set to 3° C./s or more. The average cooling rate fromthe annealing temperature to at least 600° C. is preferably 4° C./s ormore. The reason for focusing on the temperature region of the annealingtemperature to at least 600° C. is that this temperature region is atemperature region that influences the amount of austenite to becomemartensite. The upper limit of the average cooling rate from theannealing temperature to at least 600° C. is not particularlyprescribed; however, from the viewpoint of energy saving of the coolingfacility, the upper limit is preferably set to 200° C./s or less.

It is also possible to employ a procedure in which cooling is performedfrom the annealing temperature to 600° C., subsequently cooling istemporarily performed to a temperature of 600° C. or less, and reheatingis performed for retaining the steel sheet in the temperature region of450 to 550° C. In this case, in the case where cooling is performed upto the Ms point or less, tempering may be performed after martensite isgenerated.

The plating step is a step of subjecting the steel sheet after theannealing step to plating treatment and after the plating treatment,performing cooling at an average cooling rate of 3° C./s or more throughthe temperature region of 450° C. to 250° C.

If the average cooling rate in the temperature region of 450° C. to 250°C. after plating treatment is too slow, there is a problem that anamount of martensite necessary to obtain the effect according to aspectsof the present invention is less likely to be generated; thus, theaverage cooling rate is set to 3° C./s or more. The average cooling ratefrom 450° C. to 250° C. after plating treatment is preferably 5° C./s ormore. The reason for focusing on the temperature region of 450 to 250°C. is that the temperatures from the plating temperature and/or theplating alloying temperature to the martensite transformation starttemperature (the Ms point) are taken into consideration. The upper limitof the average cooling rate of the region from 450° C. to 250° C. afterplating treatment is not particularly prescribed; however, from theviewpoint of energy saving of the cooling facility, the upper limit ispreferably set to 2000° C./s or less.

Galvanization is performed by, for example, immersion in a hot-dipgalvanization bath. Hot-dip galvanization treatment may be performed bya usual method, and adjustment is made so that the coating weight perone surface is in the range mentioned above.

Alloying treatment of galvanization may be performed immediately aftergalvanization treatment, as necessary. In this case, the steel sheet isheld in the temperature region of 480 to 580° C. for approximately 1 to60 seconds.

From the viewpoint of reducing the amount of diffusible hydrogen, it ispreferable to further include, after the annealing step or after theplating step, a post-treatment step of performing heating in atemperature region of 50 to 400° C. for 30 seconds or more in anatmosphere with a hydrogen concentration of 5 vol % or less and adew-point temperature of 50° C. or less. The post-treatment step ispreferably performed as the next step after the annealing step or theplating step.

If the hydrogen concentration and the dew-point temperature of thepost-treatment step are too high, conversely there is a concern thathydrogen will be likely to enter the interior of the steel and theamount of diffusible hydrogen in the steel prescribed in according toaspects the present invention will be more than the upper limit; thus,it is preferable to create an atmosphere with a hydrogen concentrationof 5 vol % or less and a dew-point temperature of 50° C. or less.

If the heating time in the temperature region of 50 to 400° C. is short,the effect of reducing the amount of diffusible hydrogen in the steel issmall, and the present step produces only an increase in the number ofsteps; thus, the heating time in the temperature region of 50 to 400° C.is preferably set to 30 seconds or more. The reason for focusing on thetemperature region of 50 to 400° C. is that it is presumed that, in thistemperature region, dehydrogenation reaction progresses more than theentry of hydrogen and that, at this temperature or more, there is aconcern that material quality and the properties of the coating layerwill be degraded.

After the plating step, a width trimming step of performing widthtrimming may be further included. In the width trimming step, an endportion in the sheet width direction of the steel sheet is sheared. Thisprovides the effect of not only adjusting the width of the product butalso reducing the amount of diffusible hydrogen in the steel bydiffusible hydrogen being removed from the shear end surface.

The manufacturing of a high-strength galvanized steel sheet according toaspects of the present invention may be performed in a continuousannealing line, or may be performed off-line. <High strength member andmethod for manufacturing same>

A high strength member according to aspects of the present invention isa member obtained by subjecting a high-strength galvanized steel sheetaccording to aspects of the present invention to at least either one offorming and welding. A method for manufacturing a high strength memberaccording to aspects of the present invention includes a step ofperforming at least either one of forming and welding on a high-strengthgalvanized steel sheet manufactured by a method for manufacturing ahigh-strength galvanized steel sheet according to aspects of the presentinvention.

The high strength member according to aspects of the present inventionis excellent in bendability; thus, can suppress cracking after bending,and has high reliability in terms of structure as a member. Further, thehigh strength member is excellent in plating ability, particularlyplating peeling resistance. Hence, for example, at the time of pressforming a steel sheet into a member, the adhesion of zinc powder or thelike to the press mold due to peeling of the galvanized layer can besuppressed, and the occurrence of surface defects of the steel sheetresulting from the adhesion can be suppressed. Thus, the high strengthmember according to aspects of the present invention has the effect ofhigh productivity during press forming.

As the forming, common processing methods such as press forming may beused without limitations. As the welding, common welding such as spotwelding or arc welding may be used without limitations. The highstrength member according to aspects of the present invention can besuitably used for, for example, automotive parts.

EXAMPLES Example 1

The studies shown in Example 1 were performed in order to find theinfluence of the amount of hydrogen in steel.

Molten steel of the chemical composition shown in Table 1 was smeltedwith a converter, and was made into a slab under the conditions of aflow velocity of molten steel at the solidification interface in thevicinity of the meniscus of the casting mold of 18 cm/s on average andan average casting rate of 1.8 m/min. The slab was heated to 1200° C.,and was made into a hot rolled coil under the conditions of a finishrolling temperature of 840° C. and a coiling temperature of 550° C. Hotrolled steel sheets obtained from the hot rolled coil were pickled, andwere then made into cold rolled steel sheets each with a sheet thicknessof 1.4 mm under the condition of a cold rolling reduction ratio of 50%.The cold rolled steel sheets were heated to 790° C. (within the range ofthe Ac3 point+20° C. or less) that is an annealing temperature byannealing treatment in atmosphere in the annealing furnace with varioushydrogen concentrations and a dew-point temperature of −20° C., werecooled up to 520° C. at an average cooling rate from the annealingtemperature up to 600° C. of 3° C./s, were allowed to stay for 50seconds, were then galvanized and subjected to alloying treatment, andwere cooled from 450° C. to 250° C. at an average cooling rate of 6°C./s; thus, high-strength alloyed galvanized steel sheets (productsheets) were manufactured.

A sample was cut out from each sheet, and hydrogen (the amount ofdiffusible hydrogen) in the steel was analyzed and bendability wasevaluated. The results are shown in the FIGURE.

Amount of hydrogen in steel (amount of diffusible hydrogen)

The amount of hydrogen in the steel was measured by the followingmethod. First, an approximately 5×30-mm test piece was cut out from theplated steel sheet, and then a router (precision grinder) was used toremove the plating on a surface of the test piece, and the test piecewas put into a quartz tube. Next, the interior of the quartz tube wassubstituted with Ar, then the temperature was raised at 200° C./hr, andhydrogen generated until reaching 400° C. was analyzed with a gaschromatograph. In this way, the amount of hydrogen released was measuredby the programmed temperature analysis method. The cumulative value ofthe amount of hydrogen detected in the temperature region of roomtemperature (25° C.) to less than 210° C. was taken as the amount ofdiffusible hydrogen in steel.

Bendability

A 25×100-mm strip test piece was cut out from each of the manufacturedplated steel sheets in such a manner that a direction parallel to therolling direction corresponded to the short side. Next, a 90° V-bendingtest was performed such that the rolling direction corresponded to aridge to be formed by bending. Striking that makes pressing against adie with a load of 10 tons for 5 seconds, with the speed of the strokeset to 50 ram/min, was performed. A test was performed by variouslychanging the curvature radius R of the tip of a V-shaped punch in unitsof 0.5 steps, and the vicinity of the ridge of the test piece wasobserved with a lens with a magnifying power of 20 to check the presenceor absence of a crack (cracking). R/t was calculated from the smallestcurvature radius R among those at which a crack did not occur and thesheet thickness of the test piece (t (mm); the value up to the onehundredths place calculated by rounding up if the one thousandths placewas 5 or more and rounding down if it was 4 or less was used), and theresulting R/t was taken as an index of bendability. The smaller thevalue of R/t is, the better the bendability is.

It has been shown that, when the amount of diffusible hydrogen in thesteel is less than 0.25 mass ppm, bendability (R/t) is stabilized and isexcellent. The conditions of inclusions, etc. of these excellent sampleswere within the ranges according to aspects of the present invention.

TABLE 1 Si/Mn Steel Chemical composition (mass %) Ac3 (Mass No. C Si MnP S N Al Ti Nb B Ca (° C.) ratio) A 0.105 1.40 2.85 0.008 0.0008 0.00350.040 0.024 0.025 0.0018 0.0003 847 0.49

Example 2

In Example 2, galvanized steel sheets shown below were manufactured andevaluated.

Various kinds of molten steel of the chemical compositions shown inTable 2 were smelted with a converter, and were cast to produce slabsunder the conditions shown in Table 3; each slab was reheated to 1200°C. and was hot rolled at a finish temperature of 800 to 830° C., and ahot rolled coil was manufactured under the condition of a coilingtemperature of 560° C. A hot rolled steel sheet obtained from the hotrolled coil was pickled, was subjected to the steps of cold rolling,annealing, plating treatment, width trimming, and post-treatment underthe conditions shown in Table 3; thus, a 1.4-mm-thick galvanized steelsheet was manufactured. Alloying treatment of galvanization wasperformed immediately after plating treatment (galvanization treatment)under conditions of 500° C. and 20 seconds. The steps of width trimmingand post-treatment were performed only in part of the manufacturingconditions.

A sample was extracted from the plated steel sheet obtained in the abovemanner, structure observation and a tensile test were performed by themethods mentioned below, and the tensile strength (TS), the amount ofhydrogen in the steel (the amount of diffusible hydrogen), bendability,and the fractions of steel structures were evaluated and measured.Further, plating ability was evaluated. The evaluation method is asfollows.

For manufacturing conditions No. 32 of Table 3, also a galvanized steelsheet (Invention Example) was manufactured under the same manufacturingconditions except that alloying treatment of galvanized layer was notperformed. As described later, the plating ability of this galvanizedsteel sheet was evaluated by the presence or absence of a bare spotdefect.

(1) Tensile Test

A tensile test was performed with a constant tensile speed (crossheadspeed) of 10 mm/min on a JIS No. 5 tensile test piece (JIS Z 2201) takenfrom the steel sheet in a direction perpendicular to the rollingdirection. The tensile strength was defined as the maximum load in thetensile test divided by the initial cross-sectional area of the parallelpart of the test piece. When the cross-sectional area of the parallelpart was calculated, the thickness was defined as the thicknessincluding that of the coating layer.

(2) Amount of in-Steel Hydrogen (Amount of Diffusible Hydrogen)

The measurement was performed by a similar method to Example 1.

(3) Bendability

The measurement was performed by a similar method to Example 1. In thisevaluation, R/t≤3.5 was evaluated as excellent in bendability.

(4) Microstructure Observation

By taking a sample for microstructure observation from the manufacturedhot-dip galvanized steel sheet, by polishing an L-cross section(thickness cross section parallel to the rolling direction), by etchingthe polished cross section through the use of a nital solution, byperforming observation through the use of a SEM at a magnification of1500 times in 3 or more fields of view in the etched cross section inorder to obtain image data, and by performing image analysis on theobtained image data, area ratio was determined for each of the observedfields of view, and average value of the determined area ratios wascalculated. The observation position was set in the vicinity of aposition located ¼ of a sheet thickness from the surface thickness.However, the volume ratio of retained austenite (the volume ratio isregarded as the area ratio) was quantified by the intensity of X-raydiffraction; therefore, there is a case of a result in which the sumtotal of the structures is more than 100%. F of Table 4 stands forferrite, M for martensite (including tempered martensite), B forbainite, and Residual y for retained austenite. The average grain sizeof ferrite was found by observing 10 grains by SEM, finding the arearatio of each grain, calculating the circle-equivalent diameter, andaveraging the circle-equivalent diameters.

In the structure observation mentioned above, pearlite and aggregationsof precipitates and inclusions were observed as other phases in someexamples.

(5) Inclusion Observation

A ridge portion of the test piece subjected to the 90° V-bending testwas forcibly broken, and a cross section of the steel sheet was observedby SEM. The compositions of inclusions existing in an outer layer of thetest piece, that is, existing from the surface on the outside of bendingto a position of ⅓ of the sheet thickness were found by qualitativeanalysis based on EDX, and oxides containing at least one or more of Al,Si, Mg, and Ca were identified; then, the longest diameter (thedimension of the portion with the longest grain width) of each of theinclusions in an image was measured, the longest diameter was regardedas the grain size, and the average grain size of the inclusions wasfound. Further, in the field of view, the distance (the nearestdistance) from any inclusion existing in an area extending from thesurface to a position of ⅓ of the sheet thickness to an inclusionlocated nearest to the inclusion was found, the distance mentioned abovewas calculated for all the inclusions, and the resulting distances wereaveraged; thus, the average nearest distance was found.

(6) Plating Ability

The surface quality (external appearance) of the manufactured hot-dipgalvanized steel sheet was visually observed, and the presence orabsence of a bare spot defect was investigated. The term “bare spots”denotes areas having a size of about several micrometers to severalmillimeters in which no coating layer exists so that the steel sheet isexposed.

Further, the plating peeling resistance (adhesiveness) of themanufactured hot-dip galvanized steel sheet was investigated. In thepresent Example, a cellophane tape was pressed against a processedportion of the hot-dip galvanized steel sheet where bending of 90° wasperformed, peeled substances were transferred to the cellophane tape,and the amount of peeled substances on the cellophane tape was found asthe counted number of Zn pieces by the X-ray fluorescence method. Asmeasurement conditions, a diameter of a mask of 30 mm, and anaccelerating voltage of 50 kV, an accelerating current of 50 mA, and ameasuring time of 20 seconds for X-ray fluorescence were used.

Plating ability was evaluated by the following criteria. The results areshown in Table 4. In accordance with aspects of the present invention,rank A, B, or C mentioned below, which has no bare spot defect, wasclassified as passed.

A: There is no bare spot defect, and the counted number of Zn pieces isless than 7000.

B: There is no bare spot defect, and the counted number of Zn pieces is7000 or more and less than 8000.

C: There is no bare spot defect, and the counted number of Zn pieces is8000 or more.

D: A bare spot defect occurs.

The plating ability of the galvanized steel sheet not subjected toalloying treatment described above was evaluated by checking thepresence or absence of a bare spot defect. Specifically, the surfacequality (external appearance) of the galvanized steel sheet was visuallyobserved, and the presence or absence of a region where plating did notexist and the steel sheet was exposed (the presence or absence of a barespot defect) was investigated by the order of approximately severalmicrometers to several millimeters. As a result of the investigation, ithas been found that this galvanized steel sheet does not have a barespot defect and has good plating ability.

(7) Measurement of amount of oxides containing at least one or moreelements selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, V, Cu, andNi and existing in outer layer portion of steel sheet within 100 μm fromsurface of underlying steel sheet

The amount of oxides was measured by an “impulse furnace—infraredabsorption method”. However, it is necessary to subtract the amount ofoxygen contained in the material (that is, the steel sheet beforeperforming annealing); thus, in accordance with aspects of the presentinvention, outer layer portions of both surfaces of the steel sheetafter continuous annealing were polished 100 μm or more, and theconcentration of in-steel oxygen was measured and the measurement valuewas taken as the amount of oxygen OH contained in the material, andfurther the concentration of in-steel oxygen in the entire sheetthickness direction of the steel sheet after continuous annealing wasmeasured and the measurement value was taken as the amount of oxygen OIafter internal oxidation. Using the amount of oxygen OI after internaloxidation of the steel sheet and the amount of oxygen OH originallycontained in the material thus obtained, the difference between OI andOH (=OI−OH) was calculated; and a value (g/m²) obtained by convertingthe resulting value to the amount per unit area of one surface was takenas the amount of oxides.

TABLE 2 Steel Chemical composition (mass %) Ac3 Si/Mn No. C Si Mn P S NAl Others (° C.) (Mass ratio) Remarks A 0.105 1.40 2.85 0.008 0.00080.0035 0.040 Ti: 0.024, Nb: 0.025, 847 0.49 Conforming steel B: 0.0018,Ca: 0.0003 B 0.115 0.65 2.75 0.007 0.0009 0.0040 0.033 Ti: 0.022, Nb:0.018, 810 0.24 Conforming steel V: 0.005, Ca: 0.0001 C 0.140 1.50 2.850.010 0.0010 0.0035 0.040 — 832 0.53 Conforming steel D 0.140 0.68 3.320.010 0.0010 0.0035 0.060 — 789 0.20 Conforming steel E 0.086 1.00 2.850.015 0.0010 0.0035 0.040 Zr: 0.042 826 0.35 Conforming steel F 0.1911.00 2.85 0.010 0.0016 0.0035 0.040 — 797 0.35 Conforming steel G 0.1401.88 2.85 0.010 0.0010 0.0050 0.040 — 849 0.66 Conforming steel H 0.1401.00 1.65 0.010 0.0010 0.0035 0.040 — 846 0.61 Conforming steel I 0.1401.00 2.85 0.010 0.0010 0.0035 0.040 Mo: 0.10, Cr: 0.20 810 0.35Conforming steel J 0.140 1.00 2.85 0.010 0.0010 0.0035 0.040 Cu: 0.25,Ni: 0.18 810 0.35 Conforming steel K 0.140 1.00 2.85 0.010 0.0010 0.00350.040 Sb: 0.010 810 0.35 Conforming steel L 0.140 1.00 2.85 0.010 0.00100.0035 0.040 Sn: 0.02 810 0.35 Conforming steel M 0.060 1.00 2.85 0.0100.0010 0.0035 0.040 — 836 0.35 Comparative steel N 0.240 1.00 2.85 0.0100.0010 0.0035 0.040 — 786 0.35 Comparative steel O 0.140 2.25 2.85 0.0100.0010 0.0035 0.040 — 866 0.79 Comparative steel P 0.140 1.00 1.26 0.0100.0010 0.0035 0.040 — 858 0.79 Comparative steel Q 0.140 0.52 3.80 0.0100.0010 0.0035 0.040 — 760 0.14 Comparative steel R 0.140 1.00 2.85 0.0100.0030 0.0035 0.040 — 811 0.35 Comparative steel S 0.140 1.00 2.85 0.0100.0010 0.0035 0.130 — 846 0.35 Comparative steel

TABLE 3 Annealing Average cooling Casting Flow Cold rolling In-furnacerate Annealing velocity of Rolling Annealing Hydrogen Dew-pointtemperature Steel molten steel *1 ratio temperature concentrationtemperature to 600° C. Plating *2 No. No. (cm/s) (%) (° C.) (vol %) (°C.) (° C./s) (° C./s) 1 A 22 45 820 5 −15 4 5 2 A 22 45 820 5 5 4 5 3 A22 45 820 5 −25 4 5 4 B 18 50 800 9 −30 5 6 5 B 18 50 800 9 −10 5 6 6 B21 50 800 9 −10 5 6 7 B 10 50 800 9 −5 5 6 8 B 18 50 700 9 −5 5 6 9 B 1850 780 15 −10 5 6 10 B 18 50 840 9 −5 5 6 11 B 18 50 800 0.5 −5 5 6 12 B18 50 800 9 −5 1 6 13 B 18 50 800 9 −5 5 1 14 B 18 50 720 9 −25 2 2 15 B16 50 800 9 −5 5 6 16 B 18 50 800 1 −5 5 6 17 B 18 50 800 12 −20 5 6 18B 18 50 800 9 −5 3 6 19 B 18 50 800 9 −5 5 3 20 B 18 50 800 9 −5 5 6 21B 18 50 800 9 −5 5 6 22 B 18 50 800 9 −35 5 6 23 C 20 50 810 9 −5 5 6 24D 20 50 760 9 −15 5 6 25 E 17 50 780 9 −5 5 6 26 F 17 50 770 9 −5 5 6 27G 17 50 780 9 −5 5 6 28 H 17 50 780 9 −10 5 6 29 I 17 50 780 9 −5 5 6 30J 17 50 780 9 −5 5 6 31 K 17 50 780 9 −5 5 6 32 L 17 50 780 9 −5 5 6 33M 17 50 800 9 −5 5 6 34 N 17 50 780 9 −5 5 6 35 O 17 50 800 9 −5 5 6 36P 17 50 780 9 −5 5 6 37 Q 17 50 780 9 −25 5 6 38 R 17 50 780 9 −5 5 6 39S 25 50 780 9 −5 5 6 Width Post-treatment trimming Hydrogen Dew-pointHeating Presence concentration temperature Temperature Time No. orabsence (vol %) (° C.) (° C.) (min) Remarks 1 Absence 0 0 100  2880Invented example 2 Absence 0 0 100  2880 Invented example 3 Absence 0 0100  2880 Invented example 4 Absence — — — — Comparative example 5Absence — — — — Invented example 6 Absence 0 0 50 5 Invented example 7Absence — — — — Comparative example 8 Absence — — — — Comparativeexample 9 Absence — — — — Comparative example 10 Absence — — — —Comparative example 11 Absence — — — — Invented example 12 Absence — — —— Comparative example 13 Absence — — — — Comparative example 14 Absence— — — — Comparative example 15 Absence — — — — Invented example 16Absence — — — — Invented example 17 Absence — — — — Invented example 18Absence — — — — Invented example 19 Absence — — — — Invented example 20Presence — — — — Invented example 21 Absence 5 0 70 0.5 Invented example22 Absence 10  0 70 0.5 Comparative example 23 Absence — — — — Inventedexample 24 Absence — — — — Invented example 25 Absence — — — — Inventedexample 26 Absence — — — — Invented example 27 Absence — — — — Inventedexample 28 Absence — — — — Invented example 29 Absence — — — — Inventedexample 30 Absence — — — — Invented example 31 Absence — — — — Inventedexample 32 Absence — — — — Invented example 33 Absence — — — —Comparative example 34 Absence — — — — Comparative example 35 Absence —— — — Comparative example 36 Absence — — — — Comparative example 37Absence — — — — Comparative example 38 Absence — — — — Comparativeexample 39 Absence — — — — Comparative example *1 A flow velocity ofmolten steel at a solidification interface in vicinity of a meniscus ofa casting mold, *2 An average cooling rate from 450° C. to 250° C. afterthe plating treatment

TABLE 4 Steel sheet material quality Inclusions *1 Amount of SteelAverage Average nearest Coating Amount of Plating diffusible structure FSteel grain size distance weight *2 oxides *3 ability hydrogen Arearatios No. No. (μm) (μm) (g/m2) (g/m²) evaluation (Mass ppm) (%) 1 A 4055 40 0.160 A 0.12 30 2 A 40 55 40 0.230 A 0.12 30 3 A 40 55 40 0.100 B0.12 30 4 B 30 50 55 0.005 D 0.11 35 5 B 30 50 55 0.120 A 0.11 35 6 B 15100 50 0.110 A 0.06 35 7 B 50 15 50 0.150 A 0.13 35 8 B 30 50 60 0.040 B0.09 100 9 B 30 50 55 0.110 A 0.44 50 10 B 20 65 55 0.180 A 0.62 5 11 B20 65 45 0.143 C 0.16 25 12 B 20 65 55 0.152 A 0.15 35 13 B 20 65 500.150 A 0.14 30 14 B 20 65 45 0.018 B 0.09 100 15 B 50 20 50 0.110 B0.14 25 16 B 20 65 50 0.160 B 0.11 25 17 B 20 65 55 0.065 A 0.24 25 18 B20 65 55 0.152 A 0.14 30 19 B 20 65 55 0.155 A 0.14 30 20 B 20 65 550.150 B 0.08 25 21 B 20 65 50 0.140 A 0.09 25 22 B 20 65 50 0.002 D 0.1125 23 C 15 70 100 0.170 B 0.13 28 24 D 35 50 55 0.015 B 0.12 15 25 E 1565 55 0.105 B 0.14 35 26 F 15 65 55 0.120 A 0.14 13 27 G 30 60 45 0.210B 0.14 28 28 H 20 65 50 0.124 A 0.14 35 29 I 20 65 55 0.154 A 0.14 25 30J 20 65 55 0.160 A 0.14 25 31 K 20 65 55 0.153 A 0.14 25 32 L 20 65 550.157 A 0.14 25 33 M 20 65 55 0.154 A 0.14 50 34 N 30 53 55 0.132 A 0.1710 35 O 35 60 45 0.240 D 0.14 33 36 P 20 65 55 0.120 A 0.14 35 37 Q 2065 45 0.009 D 0.14 10 38 R 35 40 55 0.152 A 0.14 25 39 S 80 25 55 0.150A 0.16 35 Steel structure Retained Others F Average M Area B Area γ AreaArea Mechanical properties grain size ratios ratios ratios ratios TSBendability No. (μm) (%) (%) (%) (%) (MPa) R/t Remarks 1 8 55 10 4 11222 2.1 Invented example 2 8 55 10 4 1 1220 2.1 Invented example 3 8 5510 4 1 1219 2.1 Invented example 4 8 55 10 0 0 1208 2.5 Comparativeexample 5 8 55 10 0 0 1209 2.5 Invented example 6 8 55 10 0 0 1211 1.4Invented example 7 8 55 10 0 0 1215 5.3 Comparative example 8 14 0 0 0 0853 1.1 Comparative example 9 10 45 5 0 0 1206 4.6 Comparative example10 6 75 20 0 0 1233 5.2 Comparative example 11 6 70 5 0 0 1248 2.2Invented example 12 35 50 5 0 10 1078 2.1 Comparative example 13 10 3030 5 5 1072 2.1 Comparative example 14 30 0 0 0 0 804 1.0 Comparativeexample 15 6 70 5 0 0 1245 3.4 Invented example 16 6 70 5 0 0 1256 1.4Invented example 17 6 70 5 0 0 1251 3.3 Invented example 18 10 60 5 0 51156 2.0 Invented example 19 6 50 15 4 1 1152 2.0 Invented example 20 670 5 0 0 1253 2.0 Invented example 21 6 70 5 0 0 1255 1.6 Inventedexample 22 6 70 5 0 0 1250 2.2 Comparative example 23 4 65 4 3 0 12802.3 Invented example 24 3 85 0 0 0 1492 3.2 Invented example 25 8 55 5 05 1103 1.8 Invented example 26 4 75 10 2 0 1460 3.3 Invented example 2710 58 11 3 0 1264 2.4 Invented example 28 8 60 0 0 5 1125 2.5 Inventedexample 29 4 70 5 0 0 1280 2.5 Invented example 30 8 70 5 0 0 1283 2.4Invented example 31 8 70 5 0 0 1135 2.4 Invented example 32 8 70 5 0 01134 2.1 Invented example 33 8 50 0 0 0 1004 1.4 Comparative example 343 85 5 0 0 1510 5.0 Comparative example 35 8 52 10 5 0 1205 2.5Comparative example 36 10 55 5 0 5 1006 2.5 Comparative example 37 3 900 0 0 1500 2.5 Comparative example 38 8 70 5 0 0 1204 4.3 Comparativeexample 39 8 55 10 0 0 1180 4.5 Comparative example *1 Inclusionscontaining at least one of Al, Si, Mg, and Ca and existing in an areaextending from a surface to a position of ⅓ of a sheet thickness, *2 Acoating weight per one surface of a steel sheet *3 Amount of oxidescontaining at least one or more elements selected from Fe, Si, Mn, Al,P, B, Nb, Ti, Cr, Mo, V, Cu, and Ni and existing in an outer layerportion of the steel sheet within 100 μm from a surface of an underlyingsteel sheet F: Ferrite, M: Martensite, B: Bainite, Retained γ: Retainedaustenite

The galvanized steel sheets of Present Invention Examples obtained bymeans of components and manufacturing conditions in the ranges accordingto aspects of the present invention had TS≥1100 MPa or more, whichindicates high strength, had R/t≤3.5, which indicates excellentbendability, and was excellent in plating ability. On the other hand, inthe galvanized steel sheets of the Comparative Examples, at least one ofthese properties was poorer than in the Present Invention Examples.

Example 3

A galvanized steel sheet of manufacturing conditions No. 1 (PresentInvention Example) of Table 3 of Example 2 was press-formed tomanufacture a member of a Present Invention Example. Further, agalvanized steel sheet of manufacturing conditions No. 1 (PresentInvention Example) of Table 3 of Example 2 and a galvanized steel sheetof manufacturing conditions No. 2 (Present Invention Example) of Table 3of Example 2 were joined together by spot welding to manufacture amember of a Present Invention Example. It has been verified that thesemembers of Present Invention Examples are excellent in bendability andplating ability and can therefore be suitably used for automotive partsor the like.

INDUSTRIAL APPLICABILITY

The high-strength galvanized steel sheet according to embodiments of thepresent invention has not only a high tensile strength but also goodbendability and good plating ability. Therefore, the high-strengthgalvanized steel sheet according to embodiments of the present inventioncontributes to environment conservation, for example, from the viewpointof CO₂ emission by contributing to an improvement in safety performanceand to a decrease in the weight of an automobile body through animprovement in strength and a decrease in thickness, in the case wherethe steel sheet is used for the frame members, in particular, for theparts around a cabin, which has an influence on crash safety, of anautomobile body. In addition, since the steel sheet has both goodsurface quality and coating quality, it is possible to actively use forparts such as chassis which are prone to corrosion due to rain or snow,and it is also possible to expect an improvement in the rust preventioncapability and corrosion resistance of an automobile body. A materialhaving such properties can effectively be used not only for automotiveparts but also in the industrial fields of civil engineering,construction, and home electrical appliances.

1-11. (canceled)
 12. A high-strength galvanized steel sheet comprising:a steel sheet having a chemical composition containing a steelcomposition containing, in mass %, C: 0.08% or more and 0.20% or less,Si: less than 2.0%, Mn: 1.5% or more and 3.5% or less, P: 0.02% or less,S: 0.002% or less, Al: 0.10% or less, and N: 0.006% or less, a massratio of a content of Si to a content of Mn in the steel (Si/Mn) being0.2 or more, and the balance: Fe and incidental impurities, and a steelstructure in which an average grain size of inclusions containing atleast one of Al, Si, Mg, and Ca and existing in an area extending from asurface to a position of ⅓ of a sheet thickness is 50 μm or less, and anaverage nearest distance between the inclusions is 20 μm or more; and agalvanized layer provided on a surface of the steel sheet and having acoating weight per one surface of 20 g/m² or more and 120 g/m² or less,wherein an amount of diffusible hydrogen contained in the steel is lessthan 0.25 mass ppm, oxides containing at least one or more elementsselected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, V, Cu, and Ni andexisting in an outer layer portion of the steel sheet within 100 μm froma surface of an underlying steel sheet immediately below the galvanizedlayer account for 0.010 g/m² or more per one surface, and a tensilestrength is 1100 MPa or more, and wherein the steel structure contains30% or more and 85% or less of martensite, 60% or less (including 0%) offerrite, 15% or less (including 0%) of bainite, and less than 5%(including 0%) of retained austenite in terms of area ratio, and anaverage grain size of ferrite is 15 μm or less.
 13. The high-strengthgalvanized steel sheet according to claim 12, wherein the chemicalcomposition further contains, in mass %, at least one of (1) to (5)below, (1) one or more of Ti, Nb, V, and Zr: 0.005% or more and 0.1% orless in total, (2) one or more of Mo, Cr, Cu, and Ni: 0.01% or more and0.5% or less in total, (3) B: 0.0003% or more and 0.005% or less, (4) atleast one of Sb: 0.001% or more and 0.1% or less and Sn: 0.001% or moreand 0.1% or less, and (5) Ca: 0.0005% or less.
 14. A method formanufacturing a high-strength galvanized steel sheet, comprising: acasting step of casting steel having the chemical composition accordingto claim 12 under a condition where a flow velocity of molten steel at asolidification interface in vicinity of a meniscus of a casting mold is16 cm/s or more, and producing a steel raw material; a hot rolling stepof hot rolling the steel raw material after the casting step; a picklingstep of pickling a steel sheet after the hot rolling step; a coldrolling step of cold rolling the steel sheet after the pickling step ata rolling reduction ratio of 20% or more and 80% or less; an annealingstep of heating the steel sheet after the cold rolling step in acontinuous annealing line at an annealing temperature of 740° C. or moreand (Ac3+20°) C. or less in an atmosphere in the furnace in which ahydrogen concentration in a temperature region of 500° C. or more ismore than 0 vol % and 12 vol % or less and a dew-point temperature in atemperature region of 740° C. or more is −25° C. or more, and thenperforming cooling at an average cooling rate of 3° C./s or more fromthe annealing temperature to at least 600° C.; and a plating step ofsubjecting the steel sheet after the annealing step to platingtreatment, and after the plating treatment, performing cooling at anaverage cooling rate of 3° C./s or more through a temperature region of450° C. to 250° C.
 15. A method for manufacturing a high-strengthgalvanized steel sheet, comprising: a casting step of casting steelhaving the chemical composition according to claim 13 under a conditionwhere a flow velocity of molten steel at a solidification interface invicinity of a meniscus of a casting mold is 16 cm/s or more, andproducing a steel raw material; a hot rolling step of hot rolling thesteel raw material after the casting step; a pickling step of pickling asteel sheet after the hot rolling step; a cold rolling step of coldrolling the steel sheet after the pickling step at a rolling reductionratio of 20% or more and 80% or less; an annealing step of heating thesteel sheet after the cold rolling step in a continuous annealing lineat an annealing temperature of 740° C. or more and (Ac3+20°) C. or lessin an atmosphere in the furnace in which a hydrogen concentration in atemperature region of 500° C. or more is more than 0 vol % and 12 vol %or less and a dew-point temperature in a temperature region of 740° C.or more is −25° C. or more, and then performing cooling at an averagecooling rate of 3° C./s or more from the annealing temperature to atleast 600° C.; and a plating step of subjecting the steel sheet afterthe annealing step to plating treatment, and after the platingtreatment, performing cooling at an average cooling rate of 3° C./s ormore through a temperature region of 450° C. to 250° C.
 16. The methodfor manufacturing a high-strength galvanized steel sheet according toclaim 14, wherein the step further comprises at least one of (1) and (2)below, (1) after the plating step, a width trimming step of performingwidth trimming, and (2) after the annealing step or after the platingstep, a post-treatment step of performing heating in a temperatureregion of 50 to 400° C. for 30 seconds or more in an atmosphere with ahydrogen concentration of 5 vol % or less and a dew-point temperature of50° C. or less.
 17. The method for manufacturing a high-strengthgalvanized steel sheet according to claim 15, wherein the step furthercomprises at least one of (1) and (2) below, (1) after the plating step,a width trimming step of performing width trimming, and (2) after theannealing step or after the plating step, a post-treatment step ofperforming heating in a temperature region of 50 to 400° C. for 30seconds or more in an atmosphere with a hydrogen concentration of 5 vol% or less and a dew-point temperature of 50° C. or less.
 18. The methodfor manufacturing a high-strength galvanized steel sheet according toclaim 14, wherein alloying treatment is performed immediately after theplating treatment in the plating step.
 19. The method for manufacturinga high-strength galvanized steel sheet according to claim 15, whereinalloying treatment is performed immediately after the plating treatmentin the plating step.
 20. The method for manufacturing a high-strengthgalvanized steel sheet according to claim 16, wherein alloying treatmentis performed immediately after the plating treatment in the platingstep.
 21. The method for manufacturing a high-strength galvanized steelsheet according to claim 17, wherein alloying treatment is performedimmediately after the plating treatment in the plating step.
 22. A highstrength member, obtained by subjecting the high-strength galvanizedsteel sheet according to claim 12 to at least either one of forming andwelding.
 23. A high strength member, obtained by subjecting thehigh-strength galvanized steel sheet according to claim 13 to at leasteither one of forming and welding.
 24. A method for manufacturing a highstrength member, comprising a step of performing at least either one offorming and welding on a high-strength galvanized steel sheetmanufactured by the method for manufacturing a high-strength galvanizedsteel sheet according to claim
 14. 25. A method for manufacturing a highstrength member, comprising a step of performing at least either one offorming and welding on a high-strength galvanized steel sheetmanufactured by the method for manufacturing a high-strength galvanizedsteel sheet according to claim
 15. 26. A method for manufacturing a highstrength member, comprising a step of performing at least either one offorming and welding on a high-strength galvanized steel sheetmanufactured by the method for manufacturing a high-strength galvanizedsteel sheet according to claim
 16. 27. A method for manufacturing a highstrength member, comprising a step of performing at least either one offorming and welding on a high-strength galvanized steel sheetmanufactured by the method for manufacturing a high-strength galvanizedsteel sheet according to claim
 17. 28. A method for manufacturing a highstrength member, comprising a step of performing at least either one offorming and welding on a high-strength galvanized steel sheetmanufactured by the method for manufacturing a high-strength galvanizedsteel sheet according to claim
 18. 29. A method for manufacturing a highstrength member, comprising a step of performing at least either one offorming and welding on a high-strength galvanized steel sheetmanufactured by the method for manufacturing a high-strength galvanizedsteel sheet according to claim
 19. 30. A method for manufacturing a highstrength member, comprising a step of performing at least either one offorming and welding on a high-strength galvanized steel sheetmanufactured by the method for manufacturing a high-strength galvanizedsteel sheet according to claim
 20. 31. A method for manufacturing a highstrength member, comprising a step of performing at least either one offorming and welding on a high-strength galvanized steel sheetmanufactured by the method for manufacturing a high-strength galvanizedsteel sheet according to claim 21.